1. Field of the Invention
The present invention is in the field of resistance weld pins intended specifically to be used for the attachment of batts or sheets of fibrous insulation to the interior surfaces of air ducts to be used in air conditioning and heating installations.
2. The Prior Art
It is known to improve the energy efficiency of air conditioning and heating installations by applying to the interior portions of metal ducting batts or sheets of fibrous insulating material, generally one to two inches thick. Typically, an adhesive is employed to connect the insulation to the duct and, to assure permanence of attachment, a multiplicity of nail-like metallic members are passed through the insulation materials, the tip portions of the nails being connected to the metal ducting.
Initially metallic devices in the nature of impact fasteners were employed for this purpose. Such fasteners are intended to bite into the metal of the duct without puncturing the same. Experience has shown, however, that the vibrational forces to which ducts are subjected in many cases rupture the bond between fasteners and duct.
More recently, the art has shifted to the use of welded fasteners wherein a pin having an enlarged head and sharpened tip is passed through the fibrous material and a resistance weld effected by passing a current through the length of the pin while the duct is connected to a ground electrode and the head of the pin to a live electrode.
Since a large number of pins are typically applied to a given length of ducting, resort has been made to automatic welding machines which store a multiplicity of pins in a hopper or like orienting device and feed the same to a welding head. The operator need merely position the head and trigger an activating device, following which the welding cycle is automatically completed by progressively advancing a pin through the batt until the tip engages the duct. Continuing downward pressure against the pin head while melting current is passed through the pin results in melting of the tip and attachment of the pin.
A suitable welding pin is disclosed in my U.S. Pat. No. 3,624,340 of Nov. 30, 1971. Apparatus for automatically attaching such pins is shown in my U.S. Pat. No. 3,835,285 of Sept. 10, 1974 and U.S. Pat. No. 3,858,024 of Dec. 31, 1974, issued to myself and Charles Giannone, as well as in pending application Ser. No. 367,247, filed Apr. 12, 1982.
While the pins and attachment devices of the above referenced patents and application have achieved substantial commercial success, it has been determined that the economy and performance thereof can be improved through the use of a welding pin as hereinafter described and claimed.
More specifically, the welding pins employed heretofore have included an elongate shank having a smooth cylindrical exterior, headed at one end and sharpened at the other. The headed end of the pin has typically been enlarged either integrally or with a separate apertured disk at the under-junction of the head and shank to afford a large contact area with the surface of the relatively fragile insulating material which is usually fabricated of glass fibers. The tip end of the pin is sharpened so as to facilitate penetration through the fibers and provide secure electrical contact with the metal duct.
Since a given installation will include many hundreds or thousands of welding pins, the cost of the pins is a factor to be considered.
The pins heretofore used have employed relatively thick shank portions, in the area of 0.15". As noted in U.S. Pat. No. 3,624,340, the tip angle is pertinent to the effectivenes of the welding procedure. If the tip angle is too acute, the weld may not result in the thickest portion of the shank abutting the surface of the metal duct, with the resultant loss of stability of the attached pin as against side forces exerted thereagainst. If the angle is too obtuse, the tip may not adequately penetrate the fiber mass, and even if such penetration is achieved, due to the relatively low resistance of the tip portion resulting from its thick cross section, rapid melt may not be achieved.
Importantly, as noted hereinabove, it has been found necessary in order to provide both the necessary tip angle and current carrying capacity, to form the welding pins with relatively thick shanks even though the strength of the shank, after attachment, is not a factor in the final installation.
Attempts to effect economies through the use of significantly smaller diameter pins, with obvious attendant savings in metal costs, have proved unsatisfactory since such thinner shanks tend to overheat in the course of welding. Where such shanks overheat, the pin body tends to bow or skew due to softening of the metal, whereby the requisite pressures of tip against duct throughout the short cycle of the welding apparatus are not developed.
In the parent application above referred to, a welding pin has been provided which comprises a relatively thin cylindrical core surrounded by a series of axially spaced annular projections. A pin of the noted sort, in addition to being less expensive than thicker cylindrical shank pins, has unexpectedly been found to be subject to effective attachment with the expenditure of lower welding energy, providing equal or superior adhesion to the duct and exhibiting a substantially lesser tendency to burn or damage the shank adjacent portions of the insulation materials.
As noted in the parent application, the effectiveness of the welding pin is considered to reside in the fact that the annular projections act in the manner of heat radiating fins. With such device it is possible to utilize lesser amounts of metal in the pin while obtaining equivalent or superior attachment of the pin to the duct and, as noted, with minimal damage to the duct material.